1. Field of the Invention
This invention relates to carbon electrodes for the metallurgical industries. It especially relates to such electrodes containing high-purity carbonaceous binders prepared by solvent refining of bituminous coal to produce an ash-free solvent refined coal. It specifically relates to such binders having sufficient oxidative and thermal stability for incorporation in existing industrical procedures for manufacturing carbon electrodes.
2. Review of the Prior Art
In general, bituminous coal is classifiable as a bitumen only in the broadest sense and is somewhat more accurately classified along with lignite, peat, and enthracite as a non-asphaltic pyrobituminen. A true bitumen, however, is reversibly fusible (meltable). Bitumens as used industrially are more accurately defined as only the components which are soluble in carbon disulfide. Bituminous coal typically has a solubility of less than one percent in carbon disulfide.
When bituminous coals are industrially heated, pyrogenous distillates known as gas-works coal tar, coke-oven coal tar, blast-furnace coal tar, and producer-gas coal tar are isolated as by-products. These tars, which vary in composition, are characteristically liquid, oily, comparatively volatile, largely soluble in carbon disulfide, and yield water-soluble sulfonation products.
When these tars are partially evaporated or distilled, pyrogeneous residues, identified as correspondingly named pitches, are isolated. These pitches, which also vary in composition, are viscous to solid, adherent to non-adherent, comparatively non-volatile, fusible, also largely soluble in carbon disulfide, and also yield water-soluble sulfonation products.
Electrodes, such as carbon electrodes used in the electrolytic process of aluminum manufacture, have long been prepared by mixing a pitch-type binder with a graded aggregate of coke, either binder or coke being derived from petroleum or coal. Upon carbonization, this pitch is converted to carbon, thereby cementing together the coke aggregate. The type and purity of the coke generally determines the end use of the electrodes. Coal-derived pitches normally used as electrode binders have had to be subjected to severe cracking of the intermediate tar by thermal or catalytic means because such pitches (as well as coal extracts where hydrogen is not used) have notoriously unstable viscosities so that a catalytic or high-temperature cracking step was needed in order to maintain a workable viscosity during industrial manufacture of either Soderberg or prebaked electrodes. This situation is believed to be caused by the presence of unstable free radicals in low-temperature tars that are stabilized only by such cracking or by the presence of hydrogen.
Such stabilization is believed to be but a portion of the processing required for making the commonly used pitch binders of the prior art including bitumens, asphalts, coal tar pitch, oil tar pitch, tars, resins, gilsonite, and the like, as taught in U.S. Pat. No. 2,998,375, by distillation, cracking, solvent extraction, concentration, and the like, this processing being necessary because of the complexity, heterogeneity, and unpredictability of the chemical structure of coals. For example, during the 1950's there was a predominant theory for electrode binders that associated the quality of a binder with the amount soluble in various solvents, e.g., benzene and quinoline, so that specifications for electrode binders are still being written that specify the maximums or minimums of certain soluble fractions.
Softening point reduction to about 32.degree.-38.degree. C. for coal-tar hard pitch (distillation residue) is taught in U.S. Pat. No. 2,297,455 by addition thereto of 40-50% of anthracene oil, benzene wash oil, coal tar oil, and the like and heating for several hours without removing the additive to produce a pitch that is comparable to petroleum asphalt as to low-temperature shock sensitivity.
Coal-tar pitch, the most commonly used binder, is classified in U.S. Pat. No. 2,683,107 into four fractions differing with respect to adhesiveness and solubility in itself and in organic solvents. The fraction which is insoluble in itself is present as suspended solids. This fraction is removed by heating the pitch to temperatures between 150.degree. C. and 400.degree. C. and filtering or by dissolving the coal tar pitch in a high-boiling aromatic solvent and filtering. The filtrate, after removal of solvent if used, may be maintained at temperatures between about 400.degree. C. and 525.degree. C. until it has a softening point between about 85.degree. C. and about 125.degree. C.
Removal of these suspended solids permits the binder, as an impregnating material, to flow into the pores of a carbonaceous body without plugging by the suspended solids so that densified carbon or graphite electrodes can be prepared. Such removal does not affect the reactivity of carbon electrodes as long as the insoluble material is less than about 15 percent by weight and is not inorganic in nature.
A coal-solution process is taught in U.S. Pat. No. 3,240,566 for removing ash from coal to produce a substantially ash-free carbon which can be used in the manufacture of carbon electrodes suitable for the electrometallurgical industries. In this process, an extracting oil is mixed with crushed and dried bituminous coal, in a weight ratio from about 1:1 to about 6:1, and the mixture is digested, separated by centrifuging, concentrated, and coked. The extracting oil is an aromatic liquid hydrocarbon creosote oil-type solvent, such as high-boiling anthracene oil fractions.
After distilling a light oil from the digested coal, two-stage centrifuging is used to remove the ash. The first overflow is solvent refined coal as a coal solution from which 75-85% thereof is removed as a creosote type aromatic hydrocarbon before coking at 550.degree.-700.degree. C. The removed hydrocarbon is suitable as the initial solvent for the digestion. Cokes made from Kentucky and Alabama coals, after calcining at 1340.degree. C., had 0.58% and 0.76% ash, respectively.
In the process of U.S. Pat. No. 3,562,783, non-caking coal is digested at 350.degree.-400.degree. C. with a solvent derived from the coal itself. About 30-40% of the coal plus about 5% of coal-reacted and polymerized solvent is recovered as a non-distillable pitch binder for making form coke which is used as part of a blast furnace charge. The binder is not stabilized against oxidative or thermal polymerization, although such pitches have notoriously unstable viscosities.
In U.S. Pat. No. 3,801,342, a process is disclosed for upgrading lignite binder pitches which are produced firstly by low temperature carbonization of lignite to form a tar and a char residue, secondly by distillation of the tar to produce lignite pitches, thirdly by delayed coking or thermal cracking of the lignite pitches to produce an oil, and fourthly by distillation of this oil to leave a pitch residue which is suitable as a binder for carbon electrodes. In this upgrading process, such low-temperature pitch residue is extracted with saturated aliphatic hydrocarbons having 7-9 carbon atoms to leave an aliphatics-free binder pitch which, when combined with lignite coke, makes carbon electrodes having lower electrical resistivity and substantially higher physical strength as compared to similar electrodes containing such aliphatics.
In the art of manufacturing carbon electrodes, the binder must be stable by itself and in mixtures with other carbonaceous materials when at elevated temperatures (150.degree. C. to 170.degree. C.) for prolonged period of time. Attempting to use an oxidatively or thermally unstable binder results in severe mechanical problems and can break down the entire manufacturing process. Such stability principally manifests itself as good viscosity stability which is important both for holding the binder in heated liquid storage and for stable rheological behavior in operations using Soderberg anodes.
Moreover, the coke must be heated at high temperatures (approximately 1,200.degree. C.) before adding the binder and manufacturing the carbon paste. This heat treatment is necessary in order to provide adequate electrical conductivity and to pre-shrink the carbonaceous material and thus prevent shrinkage and distortion within the formed electrode. In the form coke process, in contrast, such pre-shrinkage is not necessary. Indeed, it is not even desirable to separate the aggregate and binder before forming the article unless the coke has such poor agglomerating properties as to require external extraction and reformation of the binder.
Because carbon electrodes are a basic necessity for the electrometallurgical industries, it is highly desirable that they can be manufactured by customary industrial procedures with any available coke aggregate and a binder that is made from a bituminous coal as a primary product, not as a by-product whose supply is based upon other industrial requirements. In consequence, a process for making such a coal-derived binder must include a means for imparting oxidative and thermal stability to the pitches so that they can be satisfactorily incorporated into existing processes for manufacturing carbon electrodes, either when used alone or as a blend with other industrially proven binders.